1. Field of the Invention
The present invention generally relates to temperature control systems and methods, and more particularly, to a preheat oven control system and method.
2. Description of the Related Art
Conventional ovens employ electric resistance heaters or heating elements in an oven cavity for baking and cooking foods. Typically, at least one heating element, referred to as a bake element, is disposed adjacent the bottom of the oven cavity. Oven cavities are generally constructed out of steel with a porcelain coating finish. As is known, the heating elements are energized to heat the oven cavity and can be controlled to achieve a plurality of different operating modes. For example, an oven may be operated in a BAKE mode or a CLEAN mode. During each of these modes, the one or more heating elements are energized to raise the temperature of the oven to a pre-determined or pre-selected temperature and then the oven cavity is maintained at the desired temperature for a period of time.
One concern in the design of ovens is to ensure that the heating elements are not orientated or operated in a manner to cause the portions of the oven cavity nearest the heating elements to experience extremely high temperatures. The surface temperature of a heating element when energized is generally within the range of 1000-2000.degree. F.--depending on the wattage density of the heating element. The porcelain finish on oven cavity walls can be degraded as a result of high temperatures and the related thermal expansion. This degradation is commonly referred to as porcelain "crazing" and is likely to occur if the oven cavity walls reach or exceed a temperature of 1000.degree. F. Accordingly, ovens must be designed so that the oven cavity walls are not subject to high temperatures which may result in porcelain crazing.
Generally it is only during the initial preheat operation of an oven for a BAKE or CLEAN mode, when the oven is heating up to the desired temperature and when the heating elements are being continuously energized that there is a risk of porcelain crazing. The maximum temperature for a typical oven is around 850-900.degree. F.--and this high temperature is only achieved during a CLEAN mode. During a BAKE mode, the oven is operated at much lower temperatures. Accordingly, it is only those portions of the oven cavity which are relatively close to the heating elements that run the risk of seeing temperatures exceeding 1000.degree. F. during a preheat period.
Many ovens avoid porcelain crazing during preheating by spacing the heating elements, and particularly the bake element, an appropriate distance above the bottom wall of the oven cavity. In this manner, even if a bake element is energized continuously for a relatively long preheat period, the oven cavity directly beneath the bake element still does not exceed 1000.degree. F.
Some oven designs, however, utilize an oven configuration wherein the bottom heating element or bake element is disposed in a separate compartment provided below the oven cavity, such as shown in U.S. Application Ser. No. 08/969,801, to Crone et al., entitled "HEATING ELEMENT SUPPORT SYSTEM FOR OVEN". These types of oven configurations may be referred to as hidden element ovens. In hidden element ovens, heat from the hidden heating element is transferred to the underside of the bottom wall of the oven cavity and is conducted throughout the entire oven cavity body and is radiated into the cavity from all of the interior oven cavity surfaces in a relatively even manner. This results in uniform heating of the oven cavity which can enhance the cooking performance of the oven. As can be readily appreciated, to achieve high efficiency in a hidden element oven, it is desirable to locate the bottom heating element close to the bottom wall of the oven cavity. This configuration, however, leads to the potential problem of porcelain crazing on the bottom wall of the oven--particularly during a preheat routine where the bake element may be energized continuously for a lengthy period.
Another way conventional ovens avoid the problems of overheating the oven liner adjacent the heating elements is to limit the watt density of the one or more heating elements. However, as can readily be appreciated, limiting the watt density of a heating element is a relatively unsophisticated method of dealing with this problem and can lead to poor performance when the line voltage supplied to the oven is lower than the target 240 V.A.C.--a situation which commonly occurs.
In addition to porcelain degradation, there is at least one additional problem which can arise during the preheat operation of a CLEAN mode. The object of a CLEAN mode is to raise the temperature of an oven cavity sufficiently to bum off food soils which have collected in the oven cavity. However, too rapid a temperature rise within the oven cavity can result in undesirable combustion and rapid expansion of gas within the oven cavity. U.S. Pat. No. 3,627,987 discloses one control method utilized to control oven cavity temperatures during the preheating of a CLEAN cycle. In the '987 patent, a thermal cycling switch is used to interrupt the energization of a heating element within an oven cavity such that the heat output of the heating element is reduced.